JP5247030B2 - Single polarization optical fiber laser and amplifier - Google Patents

Description

Explanation of related applications

  This patent application is related to US Patent Application No. 60/479892, filed Jun. 19, 2003, assigned to the same assignee and patent attorney number SP03-088P.

  The present invention relates generally to optically active devices, and more particularly to single polarization fiber optic lasers or amplifiers.

  Rare earth element doped fiber lasers such as ytterbium doped fiber lasers have applications in fields such as material processing, product marking and engraving and microfabrication. Ytterbium-doped fiber lasers operating at high power, narrow line width and high pulse energy are being developed. The application area of fiber lasers will be broadened by the availability of different operating wavelengths and even higher output power. Each could be achieved by nonlinear wavelength conversion and coherent coupling of several fiber lasers. The linear polarization output required for many of these applications has been less studied than other attributes of fiber lasers.

  Therefore, the availability of linearly polarized light with a stable polarization direction for lasers and amplifiers may be useful and even necessary in some applications. For linear polarization or single polarization, it is desirable to obtain an optically polarized (PZ) fiber that receives random elliptically polarized input light and provides polarized output light having only a single polarization. Polarization characteristics (single polarization) are suppressed by propagating only one of the two polarizations whose polarizations are orthogonal to each other, and at the same time increasing the transmission loss of the other polarization. Such single polarization fibers generally have an azimuthal asymmetry of the refractive index profile. Single-polarized optical fibers are useful for ultra-high-speed transmission systems, or for use as coupler fibers used with and connected to optical components (lasers, EDFAs, optical instruments, interferometric sensors, gyroscopes, etc.) It is. Single polarized lasers or linearly polarized lasers can be used to obtain linearly polarized single transverse mode light emission useful in a wide variety of fields. These areas include telecommunications, optical transmission, instrumentation, spectroscopy, medicine, chemical species detection and telemetry. Like some more specific examples, linearly polarized fiber amplifiers (LPFAs) are optical pumping of all or part of PZ fibers doped with rare earth dopants, replacing conventional erbium doped fiber amplifiers (EDFAs). When used with a fiber optic gyroscope, interferometric fiber sensor, nonlinear frequency conversion, polarization multiplexing and use in most phase or amplitude modulator configurations, it is orthogonal to one linear polarization state Has a much higher gain. By having such a polarization fiber laser or amplifier, known polarization beam multiplexing is used to combine the two beams into a single output with different polarization modes as long as they are orthogonally polarized. Increased multiplication capability can be achieved by using PBM.

  Some improvement in the polarization performance of single-mode optical waveguides has been achieved by stretching or distorting the symmetric fiber core as a means of separating different polarizations. However, the non-circular shape and associated stress-induced birefringence alone is generally the desired single polarization for use as an improved fiber laser or fiber amplifier or for polarization beam multiplexing for improved power multiplication. Not enough to maintain.

  Therefore, it is an ongoing development field to obtain a fiber laser or amplifier that provides a single polarization that is sustainable and sufficient for power multiplication by the PBM.

  Further, there is a need for a single linearly polarized (SP) fiber laser oscillator or amplifier that is robust and stable against external disturbances. Robust and stable means a device that maintains a single linear polarization.

  To avoid nonlinear effects such as Raman and Brillouin scattering, it is also necessary to create an SP fiber laser oscillator or amplifier with a large effective area for high power applications.

  In order to be able to multiply the optical power using the coherent beam combining technique, a stable linear SP fiber oscillator and amplifier are required.

  Finally, SP fiber laser oscillators and / or amplifiers are required to avoid temporal instabilities caused by nonlinear coupling of simultaneously propagating orthogonal polarization modes at high output power.

  The following definitions and terminology are commonly used in the art.

  Refractive index profile—The refractive index profile is the relationship between the refractive index (Δ%) and the optical fiber radius (measured from the center line of the optical fiber) over a selected section of the optical fiber.

  Radius-The radius of the section of the optical fiber is generally defined in terms of the composition of the refractive index of the material used. For example, the central core has a zero inner radius because the first point of the core section is on the centerline. The outer radius of the central core section is the radius drawn from the center line of the waveguide to the final point of the refractive index of the central core having a positive Δ. For a section having a first point off the center line, the radius from the center line of the waveguide to the position of the first refractive index point is the inner radius of the section. Similarly, the radius from the center line of the waveguide to the position of the final refractive index point of the section is the outer radius of the section. For example, the down-doped annular section surrounding the central core will have an outer radius at the interface between the annular section and the cladding layer.

The term relative refractive index percent Δ% -Δ% is the formula:
Δ% = 100 × (n _i ² −n _c ² ) / 2n _i ²
Represents the relative scale of refractive index defined by. Here delta% is the maximum refractive index of the refractive index profile compartments, denoted i, the reference refractive index n _c is taken as the refractive index of the cladding layer. Every point in the compartment has an associated relative refractive index measured with respect to the cladding layer.

The term alpha profile-alpha profile is an expression where b is the radius:
Δ (b)% = [Δ (b ₀ ) (1- [αb−b ₀ α / (b ₁ −b ₀ ) ^α ])] × 100
Therefore, it refers to the refractive index profile of the core expressed by Δ (b)%. Where b ₀ is the maximum point of the core profile, b ₁ is the point where Δ (b)% is zero, and b is in the range from b _i to b _f , ie, b _i ≦ b ≦ b _f Where Δ% is defined by the above equation, b _i is the start point of the alpha profile, b _f is the end point of the alpha profile, and α is a real exponent. The start and end points of the alpha profile are selected and entered into the computer model. As used herein, if the alpha profile is followed by a step index profile, the starting point of the α profile is the intersection of the α profile and the step profile. In the model, to smoothly connect the α profile with the profile of the adjacent profile section, the above equation:
Δ (b)% = [Δ (b _a ) + [Δ (b ₀ ) −Δ (b _a )]
× {(1- [αb−b ₀ α / (b ₁ −b ₀ ) ^α ]) × 100
It is written. Here, b _a is the first point of the adjacent section.

  In accordance with an embodiment of the invention, an optical waveguide having linear birefringence and linear dichroism is initially designed and / or measured to determine a single polarization wavelength range. An active dopant is then doped into the core of the optical waveguide having linear birefringence and linear dichroism to provide waveguide operation in the operating wavelength range that overlaps the single polarization wavelength range.

  Fill with air near the elliptical core of an optical waveguide with linear birefringence and linear dichroism so that the waveguide shows a polarization dependent loss (PDL) difference greater than 70 dB in the wavelength band of single polarization operation It is preferred that a hole which has been evacuated or evacuated is arranged. The optically active optical waveguide having linear birefringence and linear dichroism formed in accordance with the present invention is optically coupled to the optically active optical waveguide having linear birefringence and linear dichroism. Have excellent utility in a single linear polarization system with a configured optical component.

  Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description, including the following detailed description and claims, along with the accompanying drawings. It will be appreciated by practice of the invention as described in the specification.

  For the purposes of illustration herein, it is to be understood that the invention contemplates various alternative configurations, unless expressly specified otherwise. It will be appreciated that the specific fibers illustrated in the accompanying drawings and described in detail below are exemplary embodiments of the inventive concepts defined in the appended claims. Accordingly, specific dimensions and other physical characteristics related to the embodiments disclosed herein are not to be considered limiting unless explicitly stated otherwise in the claims. Similar functions of similar elements are referred to by the same numbers.

  Referring to FIG. 6, the wavelength spectrum of an optically active single linear polarization device in accordance with the teachings of the present invention is shown. An undoped single-polarization fiber 30 ′ fused to an optical waveguide having linear birefringence and linear dichroism, such as fiber 30 of FIG. 1, or a doped elliptical core in the fiber section of FIG. In the first mode and the second mode, light having a polarization component associated with the linearly polarized wave eigenmode 45 and a polarization component associated with the second linearly polarized wave eigenmode 50 are accumulated over a sufficiently long waveguide length. With a sufficient polarization dependent loss (PDL) difference between them (PDL is approximately greater than 3 dB), the first polarization mode has a first 3 dB attenuation at the first cutoff wavelength 601, The second polarization mode has a second 3 dB attenuation at the second cutoff wavelength 602 and a single polarization center wavelength between the first cutoff wavelength and the second cutoff wavelength. Polarization wavelength range 48 is given Thus, the first cutoff wavelength is smaller than the second cutoff wavelength.

  As can be seen in FIG. 1, a plurality of active dopants 90 provide an optical waveguide 30 having linear birefringence and linear dichroism to provide waveguide operation in an operating wavelength range 650 having a central operating wavelength. The single polarization wavelength center wavelength is sufficiently close to the central operation wavelength so that it is disposed in the region 34 and the operation wavelength range overlaps the single polarization wavelength range 48. Although the single polarization wavelength range 48 is shown narrow for a particular application of a Yb-doped fiber laser in which the gain 650 of FIG. 6 occurs at 1020 to 1100 nm, the operating wavelength range 650 is generally the single polarization wavelength range. It can be wider or narrower than 48. Ideally, the center wavelength of the operating wavelength range 650 should match the center wavelength of the single-polarization wavelength range 48, but each center wavelength is such that the two wavelength ranges 650 and 48 are at least an optically active single line. The polarization devices can be sufficiently close to each other to overlap the operating wavelength of the output signal 66 of FIG. Thus, the optically active single linear polarization device will oscillate or amplify within the single polarization wavelength range 48 depending on the waveguide design parameters.

  As used herein, the term “linear birefringence” means that two main states of propagation of the medium are linearly polarized waves, and such two linearly polarized states have different propagation constants. It means having an effective real part (refractive index). Another limitation of “linear dichroism” means that it also has imaginary parts (losses) of propagation constants with different polarization states.

  The propagation constant is the logarithmic rate of change for a given direction distance of the complex amplitude of any electric field component for an electromagnetic field mode that varies sinusoidally with time at a given frequency. The real part of the axial propagation constant for a particular mode is called the attenuation constant. The attenuation coefficient for the mode power is twice the attenuation constant.

  Polarization is generally defined with respect to radiant light and represents the limitation of oscillation of a magnetic or electric field vector with respect to a single plane. In the electromagnetic radiation beam, the polarization direction is the direction of the electric field vector (the electric field vibrates in the forward and reverse directions, so that positive and negative are not distinguished). Near any given stop in space, the polarization direction of the beam can vary randomly (non-polarized beam) and can remain constant (plane polarized or linearly polarized beam) Alternatively, it can have two coherent plane polarization elements, each of which is perpendicular to the polarization direction. In the case of plane polarization, all the electric field vectors of the light beam oscillate in a fixed single plane. In the case of two coherent plane polarizations, depending on the amplitude of the two waves and their relative phase, the combined electric field vector draws an ellipse and the electromagnetic wave is said to be elliptically polarized. The elliptically polarized wave and the plane polarized wave can be converted into each other using a birefringent optical system.

  Birefringence generally refers to the separation of a light beam into two diverging rays, commonly known as ordinary and extraordinary rays, along the fast and slow axes when the light beam enters a birefringent object. .

  Dichroism is defined as the selective absorption of light waves that oscillate in a certain plane relative to the propagation axis of the waveguide with respect to anisotropic materials. Light rays that oscillate in a plane perpendicular to the propagation axis are not absorbed. Anisotropy refers to a feature of a material that exhibits different properties along different propagation axes or for different polarizations of traveling waves.

  Therefore, polarization dependent loss (PDL) is a loss in a waveguide having linear birefringence and linear dichroism in which the polarization state of the propagation wave changes in the waveguide, and is between the maximum loss and the minimum loss. It is expressed in decibels as a difference.

  A “high birefringence” fiber, “polarization-maintaining” fiber, “polarization (PZ)” fiber, or “polarization asymmetric” fiber is a type of waveguide having “linear birefringence and linear dichroism”. Refers to a polarization maintaining (PM) fiber. In general, any highly birefringent fiber is a polarization maintaining fiber. For linear birefringence, the so-called “beat length” of the optical fiber is measured, and for linear dichroism, the PDL of the fiber is measured.

  High birefringence is another term for polarization preservation or polarization maintenance. In a birefringent material, the refractive index changes with the vibration direction of the light wave. The direction having a low refractive index is the fast axis, and the direction having a high refractive index perpendicular to the fast axis is the slow axis. A polarization maintaining or polarization maintaining (PM) fiber is defined as a single mode fiber that retains the plane of polarization of light entering the fiber while the beam propagates through the length of the fiber. Polarization is preserved by introducing polarization asymmetry of shape (shape birefringence) or internal stress (stress-induced birefringence) into the fiber structure. Because of this asymmetry, two mutually orthogonal polarization modes transmitted in the fiber have different propagation constants, reducing cross coupling between each other compared to a normal single mode fiber. In a polarization-maintaining or polarization-maintaining optical fiber, the element used to introduce birefringence is referred to as a stress application portion (SAP). SAP is heavily doped to give a different coefficient of expansion than the rest of the fiber material, and when the drawn fiber cools, the cross coupling between two orthogonal polarization modes transmitted in the fiber Residual stresses that limit The SAP can be configured as an elliptical or rectangular cladding layer surrounding the fiber core, or a pair of SAPs can be placed on either side of the core.

  Thus, the PM or PZ fiber has acceptable linear birefringence such that the loss of the second polarization mode 50 is zero and the loss or attenuation of the first mode 45 is 3 dB at the first cutoff wavelength. And to be considered a waveguide with linear dichroism, it must have sufficient polarization dependent loss (PDL) over a single polarization wavelength range or window 48. The loss of the second mode 50 is 3 dB at the wavelength expressed as the second cutoff wavelength. For wavelengths in the range 48 between the first cutoff wavelength and the second cutoff wavelength, the first mode suffers substantially more than 3 dB while the second mode 50 loses less than 3 dB. receive. The wavelength range 48 is referred to as a single polarization wavelength window.

  In accordance with the teachings of the present invention, by designing the waveguide parameters of a waveguide having linear birefringence and linear dichroism as represented by fiber 30, is it matched to a single polarization range 48? Alternatively, a single linear polarization device for operation in the overlapping operating wavelength range 650 is obtained. A single polarization is to be understood throughout the present invention as a single linear polarization, and when referring to the fiber 30 hereinafter, for simplicity, PZ or PM fiber has linear birefringence and linear dichroism. It means a waveguide having both.

  A double clad fiber or dual clad fiber is an optical fiber that exhibits a wide transmission bandwidth and low bending loss for reduced waveguide modes as a result of strong confinement in the high index outer clad layer and core region. In one possible embodiment, a double-clad fiber with linear birefringence and linear dichroism can be used to form a single linear polarization device.

  The resulting single linear polarization device is a fiber itself with optically active linear birefringence and linear dichroism, or a laser as the most common luminance conversion application incorporating such a fiber. Or an amplifier.

  As is known, LASER is an acronym for light amplification by stimulated emission of radiation. A laser is a cavity filled with a laser-acting material with planar or spherical mirrors at both ends. Such a material is any material whose crystals can be excited to a metastable state by light or discharge-crystals, glasses, liquids, dyes or gases. The light emitted by an atom as it falls back to the ground state excites another nearby atom, so that the light intensity is continuously increased while the light travels back and forth between the mirrors. If one mirror is made to transmit 1 to 2% of the light, a high intensity beam of coherent light with strong monochromaticity is emitted through the mirror. If a plane mirror is used, the beam is strongly collimated. According to the concave mirror, the beam appears to emerge from a point source near one end of the cavity.

  Optical pumping is a process in which the number of atoms or atomic systems in a set of energy levels is changed by the absorption of light entering the material. This process can raise the atom to a particular high energy level and create an inversion distribution between several intermediate levels. A material that emits coherent light as a result of induced transition of electrons or molecules to a low energy state in the laser is referred to as a lasing medium or an active laser medium.

Fluorescence is radiation by light of longer wavelengths or other electromagnetic radiation material as a result of absorption of some other radiation having a shorter wavelength, and radiation continues only while the stimulus that produces the fluorescence is maintained. In other words, fluorescence is an emission phenomenon that lasts for less than about ^10-8 seconds after excitation.

  A fiber laser is a laser in which the lasing medium is an optical fiber doped with a low level of rare earth halide to enable optical amplification. The output can be tuned over a wide range and can be broadband. Since the threshold power of fiber lasers is low, laser diodes can be used for pumping and no cooling is required.

  When a single-clad or double-clad linear birefringence and linear dichroic fiber is optically pumped in one embodiment of the fiber laser, the absorption of some other short wavelength radiation by the photoactive dopant As a result of this, the emission of longer wavelength light or other electromagnetic radiation within the operating wavelength range 650 due to fluorescence is used to form a single-clad or dual-clad single linearly polarized fiber laser or amplifier. be able to.

  The operating wavelength range 650 of FIG. 6 is referred to as a gain curve or gain band for the laser and an amplification curve for the amplifier. In general, gain or amplification is the increment of a signal transmitted from one point to another through an amplifier. A material that exhibits gain rather than absorption at a certain frequency for the signal to pass through is known as an active medium. In both amplifiers and lasers, the active medium is a medium in which stimulated emission will probably occur rather than absorption of light at a given operating wavelength. It is essential that such a medium has a state known as an inversion distribution, i.e. at least one quantum transition in which an energy level is more densely occupied than a lower level.

  That is, the single-clad or double-clad linear birefringence and linear dichroic fiber 30 has an elliptical core to form a single-polarization Er-doped fiber amplifier, as shown in FIG. The Er ions 90 could be doped and designed to operate in the single polarization wavelength range 48 of the amplifier. Although the single polarization wavelength range 48 and the overlying operating wavelength range or gain bandwidth 650 are shown for specific wavelengths for a Yb fiber laser, these ranges are not limited to other operating wavelength ranges and overlying single polarization ranges. Of course, it can be applied over the wave wavelength range. In particular, a single polarization fiber laser can be implemented as a three-level laser or a four-level laser, or an interband quasi-3 for operation that keeps the eye safe in a 1.5 μm spectrum with an Er dopant. Even a level laser can be implemented. Pumping light of about 920 nm or 980 nm can be used for 1060-1080 nm 4-level Yb laser oscillation.

  A single layer clad or double layer clad fiber 30 having linear birefringence and linear dichroism would be used to amplify the light input in the operating wavelength range 650. The erbium (rare earth element) ions 90 in FIG. 1 are generally added to the fiber core material as a dopant at a level of several hundred ppm. Therefore, the fiber will be very transparent at an erbium lasing wavelength of 2-9 μm. When optically pumped by a laser diode, an optical gain is generated in the operating wavelength range, and amplification occurs.

  Single-clad or dual-clad pumped fibers or amplifiers are known for normal four-level transitions or more complex three-level transitions, but single linearly polarized dual-clad fiber lasers or amplifiers are It was not known until.

  All dual fiber lasers have the property of propagating through the inner cladding layer, being linearly polarized when incident, and rapidly depolarizing when pumping light propagates several meters. This makes it impossible to obtain single polarization oscillation with pumping light induced polarization dependent gain. Therefore, a means for introducing PDL is necessary. Increasing the doping level (ie, increasing the fiber core refractive index delta) increases only the single polarization window, not the polarization dependent loss (PDL) of the fiber. What is needed is high birefringence and asymmetry, which results from the elliptical core shape and hole or aperture arrangement in the dual hole fiber of FIG. In another design, high birefringence and asymmetry can be provided, for example, from an asymmetric flat cladding layer.

Referring to FIG. 1, an optical active fiber, luminance converter, fiber amplifier, fiber laser, dielectric waveguide laser or amplifier of the present invention is shown in FIG. 1, with the same or functionally similar components having the same numbers. Are generally described and illustrated herein with reference to several exemplary or representative embodiments. In general, a double cladding structure that could be used as a fiber laser or as an amplifier has two cladding layers 32 and 36. The first (inner) multimode cladding 32 acts as a multimode pumping core. A first cladding layer or cladding 32 is adjacent to the single mode core 34 and a second cladding 36 surrounds the first cladding 32. The first multi-mode cladding or inner cladding layer 32 is preferably about 0.2 to the input pumping light 64 that is coupled from the active pumping source 72 by the optical system used as needed, such as the lens 70. It acts as a waveguide with a high numerical aperture (NA _cladding ) in the range of ~ 0.5. It is known that a replica fiber laser will work directly as a pumping source 72 to form an amplifier without a lens 70 because the fibers are mode matched with high efficiency to their similar dimensions.

The cross section of the first multimode cladding (D _cladding is the major axis dimension A ′ of the inner cladding layer as seen in FIG. 3) is, for example, the near-field shape of the pumping source 72 (D _laser is a large area laser beam) It can be designed to have a desired shape, matched to any other mechanism or shape that increases the coupling efficiency of the pumping beam 64 (which is the size of the radiating aperture 42). The numerical aperture (NA _cladding ) between the first cladding layer and the second cladding layer must be large enough to capture the output 64 of the pumping laser diode 72. The actual increase in luminance depends on the cladding-to-core ratio (CCR) of the pumping cladding layer area to the core area, the higher the ratio (CCR), the larger the luminance increase. However, since the absorption of pumping light is inversely proportional to this ratio (CCR), a longer device length is required as the area separation between the core cross section and the clad layer cross section increases. The ratio of pumping clad layer area to appropriately high core area (CCR) generally has a lower level of inversion that can be achieved with a given pumping power as the ratio (CCR) is higher. This makes it difficult to achieve a high inversion level, but does not become a significant obstacle to four-level laser oscillation. That is, pumping light absorption and inversion are related.

  Thus, the use of rare earth elements such as Er, Yb, or Nd as the dopant 90 in the dual clad fiber amplifier / laser core 34 with a high clad to core ratio (CCR) is problematic. Even with the very high power available from the diode laser bar, it is very difficult to reach the high inversion level required for operation of a three-level system for a laser or amplifier.

To obtain gain, a high inversion of> 50% is required at the three level transition. In the quasi-three level transition, although it is low, a considerably high inversion level is required as compared with a four-level laser in which gain can be obtained by minimal inversion. Yttrium ions and neodymium ions (Yb ⁺³ and Nd ⁺³ ) provide a three-level laser oscillation system at about 980 nm and a quasi-three level laser oscillation system at about 940 nm, respectively. In a three-level system, lasing occurs between a ground state from the excitation level or a state that is not separated by more than a few kT from the ground level (ie, thermally mixed at the operating temperature). As a result, the lasing power threshold can be a problem due to the unpumped doped core exhibiting strong absorption at the laser wavelength and insufficient inversion distribution.

  In the case of competing high gain 4 level transitions, for example at about 1060 nm for neodymium (Nd), the doped core remains transparent at the laser signal wavelength when not pumped. As a result, the power threshold for lasing depends essentially on the doped core and inner cladding layer dimensions of the double clad fiber structure and the background loss of the double clad fiber over the pumping light absorption length.

Similarly, the Yb ⁺³ ion has gain in a narrow 6 nm wide pure three-level transition from 976 to 978 nm, showing considerable promise for ytterbium as a pumping light source for high power pumped EDFAs, but for transparency Because of another competitive high gain transition of Yb with a peak at about 1030 nm (but extending to 1120 nm), which is a wide quasi-4 level transition that only requires a few percent inversion distribution, high efficiency Yb976 nm fiber lasers are still Not realized. Therefore, the reason why it is difficult to achieve laser oscillation at 980 nm (or close to 976 nm) means that the laser oscillation power threshold is also high, requiring a high inversion level (> 50%) for the 976 nm transition. It is. In addition, the competitive quasi-4 level transition at about 1015-1030 nm of Yb produces amplified spontaneous emission (ASE) that saturates the inversion, so that sufficient inversion at about 976 nm is difficult to achieve.

The above inversion problem arises from the relationship between the gain and pumping light absorption in the two competitive transitions of Yb. As a representative example, the gain at two wavelengths (assuming uniform spread) in Yb-doped germano-aluminosilicate glass is given by equation (1):

Are related. Where G ₁₀₃₀ and G ₉₇₆ are the gains at ₁₀₃₀ nm and ₉₇₆ nm, respectively, α _P is the partial amber pumping light absorptance in decibels (dB) units, and Γ _S and Γ _P are the signal modes and pumping modes of the dopant profile. Each overlap factor.

A similar relational expression with different coefficients would hold for another host, such as antimony silicate glass. As is known, double clad fibers allow coupling from diode bars and other similar active structures. However, this usually increases the pumping light overlap due to the doping profile for signal overlap, since the doping needs to be localized at or near a single core in order to obtain sufficient optical gain for the core mode at the signal wavelength. Achieved by decrementing. In general, the core is uniformly doped and the area ratio (CCR) between the pumping optical waveguide and the signal core is on the order of 100: 1 for a typical double clad fiber laser. This results in Γ _S = 1 and Γ _P <0.01. If these values are used in the equation (1), a gain of about 20 dB is generated for each pumping light absorption of 1 dB at 1030 nm. Similarly, for an overlap ratio of Γ _S / Γ _P = 50, a gain of up to 36 dB at 1040 nm will occur for every 1 dB of pumped light absorption.

  Inevitably, the higher the competitive transition gain, the higher the level of amplified spontaneous emission (ASE) that saturates the inversion. Even with weak pumping, the ASE at 1030 nm will saturate the amplifier, depleting or otherwise inhibiting the accumulation of inversion profiles required for lasing at 976 nm. In fact, even without an optical cavity, laser oscillation at a longer four-level wavelength is possible only by backscattering. Thus, high pumping light absorption would be advantageous for gain at 1030 nm or longer, even if the laser mirror defining the cavity is tuned to 976 nm.

  That is, in a quasi-three-level or three-level clad layer pumped fiber laser, the required inversion level is relatively low (<5%) due to the small overlap with the doped region of the pumping light power spatial distribution. The gain of the laser transition is considerably higher. Therefore, in order to achieve a desired three-level or quasi-three-level oscillation at a necessary inversion level, it is necessary to suppress the gain of these competitive transitions.

  Creating a sufficiently long fiber length for a constant pumping light power is equivalent to reducing the mean inversion, so one conventional approach avoids lasing at the quasi-four level transition at about 1030 nm, Instead, the fiber laser length was shortened sufficiently to give priority to laser oscillation at 980 nm. However, short fiber lasers are less efficient.

In accordance with the teachings of the present invention, only in the case of the Yb3 level transition at 980 nm, Γ _S and _D are closely related to the area ratio (CCR) of the inner cladding layer area (A _cladding / A _core ) corresponding to the _core area. it can be used equation (1) to estimate the desired overlap ratio of gamma _P. If at least 6 dB absorption of pumping light is desirable with respect to Yb, and more than 40 strong dB gain in the competitive quasi-4 level transition at 1030 nm cannot be suppressed, the desired value can be obtained using equation (1). A _clad / A _core can be calculated. That is, for a preferred host silicate glass, the desired cladding to core ratio (A _cladding / A _core ) is found to be less than 8 for a Yb double cladding fiber laser.

Therefore, an optically active polarization (PZ) fiber 30 for single-polarization operation is optimally designed specifically for the creation of single-polarization fiber lasers or amplifiers. The For more difficult three-level configurations, the double-clad active fiber 30 is doped with photoexcitable ions 90 having a three-level transition or any other type of ion that requires a high inversion level, It has a doped center or core 34. The core 34 has a core refractive index (n _core ) and a core cross-sectional area. The cross-sectional area can be calculated from the dimensions A and B of the core 34. The inner cladding layer 32 surrounds the core 34. The inner cladding layer 32 has an inner cladding layer refractive index (n _{inner cladding} ) smaller than the core refractive index, and an inner cladding layer sectional area 2 to 25 times the core sectional area (2 <CCR < 25) and having an aspect ratio greater than 1.5: 1. This preferred structure and size of the double clad active fiber 30 allows strong pumping light absorption greater than 6 dB while simultaneously suppressing long wavelength ASE. The inner cladding layer cross-sectional area can be calculated from the dimensions of the inner cladding layer, including the longer dimension A ′, as taught by the present invention and illustrated by FIG.

  Referring again to FIG. 2, the outer cladding layer 36 surrounds the inner cladding layer 32 and has an outer cladding layer refractive index that is less than the inner cladding layer refractive index.

  An example of a Yb fiber laser using fiber 30 for operation at 1060 nm would be a simple quaternary four level laser implementation than a complex three level laser. In contrast to operation as a three-level laser, operation as a four-level laser should have no limit on the inner cladding to core area ratio (CCR).

However, for the more difficult use of active PZ fiber 30 containing photoexcitable Yb ions with a three-level transition, cavity and fiber optimization must be performed. A mirror 60 having a signal reflectivity of 100% and transparent to the pumping light is disposed at the pumping end of the active fiber 30. An output mirror 62 provided as necessary gives a signal reflectivity of about 4% to the output end. If the waveguide loss is ignored, G ₉₇₆ = 7 dB. It is desirable that at least 6 dB of the pumping light power is absorbed, but it is desirable that the 1040 nm gain be suppressed to 40 dB or less by wavelength selective feedback. By substituting these values into equation (1), a preferred cladding to core area ratio or overlap ratio Γ _S / Γ _P can be obtained, and 7.6 for the rare earth dopant Yb for use in a Yb fiber laser at 980 nm. The maximum ratio is obtained.

  The photoexcitable ions 90 could be one or more of a transition metal, such as chromium, in addition to one of the rare earth elements. If an element such as Ge, P, or B is used to give the fiber a Raman gain, any photoexcitable ion for use as a double clad fiber laser for pumping a fiber with Raman gain is suitable. It is a rare earth element.

In general, the active fiber 30 can be used as an amplifier or a fiber laser. For all rare earth dopants 90 that require all, especially high inversion levels, as photoexcitable ions, such as Er, Nd, Tm, and Yb, the present invention provides a maximum allowable inner cladding layer for a double cladding structure. Teach area. Generally available from any type of laser diode, assuming pumping light absorption cross section (σ _ap ), metastable level lifetime (τ) and desired average inversion level (n 2), and specific power absorption Given pumping power, input and output (non-absorbing) pumping optical power values can be estimated as P input and P output, respectively, as taught by the present invention for any rare earth and host material system. And formula (2):

Can be used to find the maximum cross-sectional area of the cladding layer. Where hν is the pumping photon energy.

  Regardless of how the ion and host materials are different, equation (2) is universally applicable and is particularly suitable for amplifiers operating at levels well below saturation. In the classical case of the active fiber 30 used as a conventional, ie C-band silica glass Er-doped amplifier (EDFA) operating at 1530 to 1565 nm, the A cladding is substituted by substituting the value corresponding to equation (2) <780 μm 2 is taught by the present invention. Thus, in general, it is not the cladding to core ratio (CCR), but the absolute dimensions of the inner cladding that is most critical to high efficiency laser or amplifier operation. Thus, the core 34 can be any size that fits inside the inner cladding layer 32. However, the core 34 is preferably similar in size and NA to the standard single mode fiber 20, which would facilitate the coupling of the output fiber 20 to a laser or amplifier. A typical single mode core radius of 3-4 μm can lead to a 10: 1-20: 1 clad to core area CCR (A clad / A core) for the case of C-band Er-doped fiber.

  In this example, the double-clad fiber amplifier is based on silica glass co-doped with Ge and Al (type II) and is pumped at 980 nm (σap = 2.55 × 10−25 m2, τ = 8 ms, hν = 2.03 × 10 −19 J). A single 2W laser diode is used to pump the amplifier. Using this 2 W power of the laser diode, 80% of the available power (P input = 1600 mW) is coupled to the inner cladding layer. Given the desired power efficiency of the fiber amplifier, more than 1/2 of the power will not leak at the other end (P output = 800 mW). For type IIC band amplifiers, an average inversion (n2) of ~ 0.6 is required to achieve the minimum gain "ripple" (gain variation within the useful amplification band). By substituting these values into equation (2), the inner clad layer cross-sectional area A clad≈780 μm 2 can be obtained. This means that for an inner cladding layer cross-section greater than 780 square μm, an average reversal of 0.6 can be achieved unless a more powerful (available power greater than 2 W) pumping laser is used. It will not be. In practice, the available dimensions of the inner cladding layer will be limited to even smaller values on the order of 500 μm 2 or less due to passive losses.

  Using a typical core radius of a = 3 μm, the clad-to-core area ratio CCR is A clad / A core = 500 / (π · 32) ≈18, which is useful for the operation of double clad lasers and amplifiers. In contrast, the values recommended in previous references or well below those reported so far.

  Thus, for a C-band Er-doped double clad amplifier pumped with a 2W 980 nm large area laser diode, the recommended values for clad to core area ratio according to the teachings of the present invention are 10: 1 to 20: 1, In this case, the cross-sectional area of the inner cladding layer should not exceed 500 μm 2. If the power available in the laser diode is doubled as in the 4W pumping diode, the recommended value is also doubled, so the clad to core area ratio is 20: 1 to 40: 1 in this case, and the inner clad layer area However, in this case, it is less than 1000 μm 2.

  For amplification in long wavelength or L-band amplifiers operating between 1570 nm and 1620 nm, a fairly small average inversion value is required, such as about 0.4. Corresponding to the decrease in mean inversion value, the maximum cross sectional area of the inner cladding layer that can be used is at least 2.5 times larger than in the case of a C-band amplifier. The operation of a double clad L-band amplifier pumped with a 1.76 W 980 nm laser diode module with an inner clad layer cross-sectional area of 2100 μm 2 is shown in practice. However, due to the circular inner cladding layer shape and small pumping light absorption of this amplifier, the amplifier efficiency was only ˜15%. Even smaller cladding layer dimensions are advantageous for both the L band and the C band, as higher pumping light absorption levels may be possible for the same average inversion. Thus, for an L-band Er-doped double clad amplifier pumped with a 2W large area laser diode, the recommended value for the clad to core area ratio CCD is 10: 1 = 50: 1, and the cross-sectional area of the inner clad layer is Should not exceed 2000 μm2.

  If the Yb fiber laser provided by the active fiber 30 is pumped by a single 2 W large area laser diode 72 for high efficiency laser operation and the input pumping power P input = 1600 mW is actually incident on the inner cladding layer 32 For example, the threshold power required for lasing should not exceed about 1/4 of the input pumping power, or about 400 mW. If αp = 6 dB, (for 920 nm pumping light) hν = 2.16 × 10-19 J, σap = 8.3 × 10-21 m2, τ = 0.8 ms and P input = 0.4 W From (4), the clad layer area is A clad = 890 μm 2. Therefore, for a Yb-doped 976 nm double clad fiber laser pumped with a 920 nm large area laser diode, the recommended value for the clad to core area ratio is 2: 1 to 8: 1 from equation (1) and the threshold is possible Since it should be lowered as much as possible, the cross-sectional area of the inner cladding layer should not exceed 900 μm 2 from equation (4).

  A double clad fiber with such a small clad to core area ratio is feasible. For a 8 μm diameter circular core in the preferred 10 × 30 μm elliptical inner cladding layer, the area ratio is (5 · 15/42) ≈4.7, which is less than the maximum ratio of 8 taught for Yb.

  The preferred structure and dimensions of the double-clad active fiber 30 allow strong pumping light absorption while suppressing long wavelength ASE, and strong pumping light intensity sufficient to obtain a three-level transition. For example, the input side of a three-level or quasi-three level double clad active fiber, i.e., luminance converter 30, for use as an amplifier or laser is illuminated with a pumping signal 64 of wavelength λP. A preferred single transverse mode core 34 centered within the multimode inner cladding layer 32 is made of glass having a composition that is sufficiently different from the inner cladding layer 32 to provide the appropriate refractive index difference. The core 34 does not have to be strictly single mode, and operates even with a core on the boundary with the two modes. Preferably for high power laser oscillation, the core 34 is doped with ytterbium ions (Yb + 3), erbium ions (Er + 3) or neodymium ions (Nd + 3), although other rare earth elements 90 can also be used. The double clad active fiber 30 also has an outer cladding layer 36, preferably made of glass having a refractive index lower than that of the inner cladding layer 32, such that the NA cladding is greater than 0.3. The total glass structure allows these types of refractive indices, and glass types include lanthanum aluminosilicate glass, antimony germanate, sulfide, lead bismuth gallate and the like. A preferred material for the overclad is also glass, such as alkali-boroaluminosilicate.

  In the cross-sectional area diagram of the active PZ fiber 30 of FIG. 2, 5, 7 or 8, the relative diameters of each are not accurately represented. However, the area of the inner cladding layer 32 is preferably approximately smaller than 25 times the area of the core 34. It is also possible to provide a single layer clad single polarization laser or amplifier using air (n = 1) as the outer clad layer.

  The length of the active fiber 30 is relatively insignificant except that it is very long compared to that wavelength so that any higher order modes are sufficiently attenuated over that length. In practice, this length is determined by the rare earth doping level of the core and the desired pumping light absorption efficiency. In some situations, a length of 1 cm is more than sufficient.

  The active PZ fiber 30 has two mirrors 60 and 62 to serve as end reflectors that define the input and output ends of the optical cavity 46, respectively. The input mirror 60 is formed so as to have an extremely high transmittance with respect to the optical pumping signal 64 at the pumping wavelength λP, and to have an extremely high reflectivity at the signal (laser oscillation) wavelength λS of the output signal 66. It is made to be reflective to some extent (transparency to some extent) at the wavelength λS, and preferably at least to some extent also to the pumping wavelength λP. For the active fiber 30 used as a fiber laser, a cleaved output facet can be used as the output mirror 62. Even a 4% reflectivity caused by an air gap to the butt coupling output fiber 20 is sufficient to define the optical cavity.

  Single mode fiber 20 is butt coupled to the output end of core 34. If the luminance converter or fiber laser 30 is used as a pumping light source for an EDFA or other doped optical amplifier, such as a Raman amplifier or fiber with Raman gain, the single mode fiber 20 amplifies the active pumping light source. A pumping optical fiber for coupling to the fiber. This allows the pumping signal 64 to enter the optical cavity 46 with high efficiency at the input mirror 60. The optical cavity 46 is defined between the mirrors 60 and 62, and some of the standing waves in the optical cavity can pass through the output mirror 62.

  In the example of an ytterbium fiber laser provided by an active fiber 30 for three-level laser oscillation, the signal wavelength λS is equal to 978 nm corresponding to the three-level Yb + 3 transition. Although the present invention related to the fiber laser was developed from the viewpoint of Yb + 3 doping, the present invention is not limited to this. The fiber laser or luminance converter 30 can be doped with another transition element ion or rare earth element ions 90, such as Nd + 3. The combined use of Yb and Nd doping, either by co-doping or by coupling differently doped fibers, allows pumping at 800 nm instead of 920 nm.

  Even without the use of a separate focusing element 70, the optical characteristics of the large area stripe laser 72 can be good enough to allow direct coupling to the multimode inner cladding layer 32. However, if the focusing element 70 is required, the pumping optical power from a large area laser diode having a radial aperture of 100 × 1 μm 2 in general and a NA of 0.1 / 0.55 in each of the slow axis and the fast axis will generally be provided. A technology has been developed that enables highly efficient coupling to fibers having a rectangular core with a cross section of 30 × 10 μm 2 and an effective numerical aperture> 0.42. The terms “slow” and “fast” refer to surfaces that are “parallel” and “perpendicular” to the junction surface of the laser diode, respectively. Light from a large-area semiconductor laser 72 having a light emitting portion size of 100 × 1 μm 2 and an NA of 0.1 / 0.55 (measured at 5% of the maximum far-field intensity point) in each of the slow axis and fast axis In order to combine with high efficiency, a coupling optical system for generating a far-field image of a light emitting part having a dimension of 30 × 10 μm 2 and 5% NA in each of the slow axis and the fast axis of 0.35 / 0.12 or others The beam shaper 70 can be designed.

  Regardless of direct coupling or not, by laser diode 72 in the form of large area stripes, arrays, diode bars of AlGaAs or InGaAs emitting at wavelengths shorter than 976 nm but in the ytterbium absorption band, or another fiber or A pumping signal can be provided by the multiplexed combination of stack diodes. The practical pumping band extends from 850 nm to 970 nm, a more preferable range is 910 to 930 nm, and a most preferable range is 915 to 920 nm. The exact values of these bands and lasing wavelengths can shift several nanometers depending on the host dielectric.

  In accordance with the teachings of the present invention, the input mirror 60 is a narrowband filter, preferably having a narrowband bandwidth centered on the single polarization bandwidth (SPB) 48 of FIG. A fiber Bragg grating (FBG). In order to use the bandwidth limitation (eg, by FBG) of the input mirror 60 to achieve single polarization or lasing in a double clad fiber laser, moderate birefringence (10-4 to 10-6) PZ fiber 30 is preferred. As is known, with the correct length of birefringence from the stress-induced region of the PZ fiber 30 or the shape-induced region (such as an asymmetric flat cladding layer) of the correct length (no dichroism) The (or small) fiber oscillator is designed to have a wavelength dependent polarization output state under the gain bandwidth for the anisotropic fiber 30. With such a suitable birefringent or dichroic region, the resulting PZ fiber 30 has polarization dependent loss (PDL), sometimes referred to as dichroism. Bandwidth limitation by the input mirror 60 is used to select a particular single polarization of the PZ fiber 30.

  The gain bandwidth limitation of the present invention is particularly important for high power operation of single polarization fiber lasers. For example, taking the dual hole structure of FIG. 2, most rare earth transitions have a gain bandwidth that is considerably wider than the single polarization bandwidth that can be achieved with a core of 0.5-1%. In general, such as by reducing the core diameter or increasing the doping level of glass components such as germanium (Ge), phosphorus (P), aluminum (Al) or boron (B) to change the core NA, By increasing Δ, the single polarization bandwidth can be easily widened, but the latter has the negative effect of reducing the effective area of the fundamental mode. For high power operation, it is required to have the maximum effective area of the fundamental mode, and one simple way to do this is to reduce Δ and increase the core radius, but if Δ is reduced The single polarization window will also be narrow, and thus it will be necessary to have a gain bandwidth limit that can be selected by a separate grating or other reflector.

  Referring again to FIG. 6, a similar plot of the output power spectrum S0 of an Nd-doped fiber laser with SP fiber 30 having birefringence in the range of 10-4 to 10-6 and a length of about 6 m is shown. This can be easily replaced with a Yb-doped fiber shown as gain bandwidth 650. With either type of rare earth dopant, the position of higher gain to align the single polarization bandwidth 48 with the fiber laser bandwidth 650, fiber Bragg diffraction of the input mirror 60 for the high power fiber laser of FIG. It can be selected depending on the grating embodiment, which is expressed in terms of the output power spectrum, transmittance or spectral intensity of the diffraction grating 60. The narrowband filter (F) of the diffraction grating 60 of FIG. 1 has a fiber laser gain bandwidth (FWHM) 650 and a bandwidth (FWHM) narrower than the single polarization bandwidth 48. The optimum filter band for Nd or Yb fiber is in the range of 1-30 nm. A critical parameter for obtaining a continuous wave is the generation spectral bandwidth 650 of the output signal from the laser or amplifier of FIG. 1, which is wider than the intracavity filter bandwidth (F). Generation of high power signals is not limited to a Fabry-Perot cavity as shown in FIG. 1, and any waveguide laser cavity structure with an intracavity waveguide and a narrow bandpass filter can be implemented. In fact, the diffraction grating 60 need not be a narrow band and can be a bandpass filter 60 'as shown in FIG. 12, or any other suitable reflector.

  As an example of the SP fiber 30, the first embodiment of the single-polarization optical waveguide fiber 30 has a cross-sectional structure as best shown in FIGS. The single polarization fiber 30 'of FIG. 12 has the same configuration except that the core is not doped with active ions. In the illustrated embodiment, the active ion doped optical waveguide PZ fiber 20 has a central core 34 that extends along the fiber axis and has a maximum dimension A and a minimum dimension B. The cross-sectional shape of the central core 34 is elongated, and is preferably approximately oval. Preferably, the elongate length is defined as A / B, where the drawn fiber 30 is greater than 1.5, preferably between about 1.5 and 8, more preferably between 2 and 5. It will be controlled during the fiber process (drawing or redrawing) to show the aspect ratio AR1.

  The central core 34 is preferably made of germanium-doped silica, the germanium being between about 0.5% and 2.5%, more preferably between about 0.9% and 1.3%, in one embodiment. Is provided in a sufficient amount such that the core has the cores Δ% and Δ1 shown in FIGS. The average diameter d average of the central core 34 = {A + B} / 2 is preferably between about 3 μm and 12 μm, more preferably between 4 μm and 10 μm.

  It has been found that by increasing the cores Δ% and Δ1, the single polarization bandwidth 48 (see FIG. 6) can be shifted to the longer wavelength side. Conversely, reducing the diameter of the dichroic region holes 24, 26, which act as a PDL difference shaper, optimizes the various fiber parameters with smaller holes and reduces the single polarization bandwidth 48 to the short wavelength side. Can be moved. The single polarization bandwidth 48 is between the cutoff wavelength of the first polarization 45 and the cutoff wavelength of the second polarization 50. Within this wavelength band 48, a true single polarization, ie one and only one polarization, is provided. Herein, the single polarization bandwidth 48 is measured at a 3 dB drop point from the linear region 49 of the plot best shown in FIG.

  As an example, referring to FIG. 6, a single polarization bandwidth (SPB) 48 spans between about 1057 nm and 1082 nm, thus providing a single polarization bandwidth of about 25 nm. However, this range is exemplary and it will be appreciated that other wavelength bands may be provided for the PZ fiber. The width of the single polarization region (SPB) can be increased by increasing the core delta and decreasing the average core diameter. Similarly, the SPB position can be adjusted as described above. Further adjustments can be made to the single polarization fiber to adjust the relative position or width of the SPB 48 (see Table 1 below).

Table 1 below shows various changes in the hole diameter (d), changes in core Δ%, Δ1, the first aspect ratio AR1 and the d-average of the central core 34 based on the model calculation. The sensitivity of the cutoff wavelength λ1 of the first polarization, the cutoff wavelength λ2 of the second polarization, and the single polarization wavelength bandwidth Δλ to the change is shown.

  Examples 1-18 above show the sensitivity of the PZ fiber 30 to changes in various structural parameters. In particular, it can be seen in Examples 1-4 that the single polarization wavelength band can be shifted to the short wavelength side by changing the hole diameter from 1 μm to 15 μm. Examples 14-18 show dramatically how the cores Δ%, Δ1 can be used to increase the single polarization wavelength bandwidth. The remaining examples show how the average core diameter d-average and aspect ratio AR1 can be used to affect the single polarization bandwidth and the relative position of the bands.

  The fiber parameters discussed so far are not the only possible design parameters for the fiber. Passive single polarization fibers have several design parameters, such as core ellipticity, core dimensions, core delta, and adjacent hole dimensions, for optimization for different applications. These fiber parameters can be designed to achieve a cutoff wavelength difference for the desired value. The single polarization window is the wavelength range between these cutoff wavelengths, so the single polarization window can also vary depending on various fiber parameters. For the use of the active single polarization fiber 30, two applications are prominent.

  First, for a single polarization amplifier, the fiber design parameters are designed so that the wavelength to be amplified falls within the single polarization wavelength region of the single polarization fiber. If the wavelength to be amplified is greater than any cut-off wavelength (ie, outside the top of the single polarization window), no polarization will naturally occur because no polarization is transmitted. If the wavelength is smaller than any cutoff wavelength, any polarization will be amplified and no single polarization function will occur. As an example, one could have a single polarization Er-doped fiber amplifier (SP-EDFA) in which an active dopant 90, which is erbium (Er), is doped into the elliptical core of the SP fiber.

  Second, for a single polarization oscillator (laser), the fiber parameters need to be designed so that the gain bandwidth matches or is narrower than the single polarization window. Otherwise, to ensure that feedback is stronger for wavelengths where single polarization would occur, for example, fiber Bragg with high reflectivity at Bragg wavelengths falling within the single polarization window It is necessary to implement wavelength selective feedback by the input mirror 60 using a diffraction grating.

  For Yb fiber lasers, gain occurs in the range of 1020 nm to 1100 nm, so single polarization (SP) fiber 30 must be designed to have a single polarization window in this range. If the gain band is much wider than the SP window 48, a highly reflective diffraction grating 60 can be used to limit the gain to only the narrow wavelength region range that falls within the SP window 48 and provide feedback (although other Any of the wavelength selective filters can also be used). The high reflectivity of the input mirror 60, preferably the diffraction grating, is desirable to extract sufficient power from one side of any high gain fiber laser. Another preferred fiber parameter is that the core 34 has an elliptical shape to provide a large mode area with a core refractive index Δ% of about 0.15% to give a numerical aperture of about 0.8.

  Referring back to FIG. 2, the central core 34 has a composition different from that of the central core, and preferably has a refractive index lower than that of the core, and is surrounded by an annular region 12, sometimes referred to as a flat cladding layer. Accordingly, the annular flat clad layer region 12 is preferably doped with a refractive index lower than that of pure silica, and is most preferably made of fluorine-doped silica. As shown in FIG. 4, the annular flat cladding layer region 12 is preferably between about -0.0% and -0.7%, more preferably between about -0.2% and -0.6%. Most preferably, Δ% and Δ2 of about −0.4% are exhibited. In general, the glass in the annular flat cladding layer region 12 is doped to have a higher viscosity than the central core 34 at the drawing temperature. The annular flat clad layer region 12 may have a generally elliptical shape as indicated by the core / cladding interface 22 in FIG. 2, and more preferably may have a generally circular shape as indicated by the dashed line 38.

  In embodiments having a circular shape, the annular flat cladding layer region 12 is preferably between about 10-15 μm, more preferably between about 13-19 μm, and in one embodiment about 16.5 μm outer diameter. D. If desired, the annular flat cladding layer region 12 can have a generally elongated shape, such as an ellipse. In this case, the average dimension D average = {A ′ + B ′} / 2 is about twice the central core 34, for example, between about 6 to 16 μm, and the second aspect ratio AR2 defined by A ′ / B ′. Is between about 1.5 and 8.

  In addition to the elliptical central core, at least one air hole is formed on each side of the core 34. The holes 24 and 26 are preferably formed at least partially in the annular flat clad layer region 12 of the fiber 30. The holes 24, 26 are preferably air filled or evacuated holes and extend along the entire axial length of the fiber 30, the dimensions of which are substantially constant along the fiber length. Is preferred. The holes 24, 26 are preferably arranged to face each other across the diameter of the central core 34, and can be formed completely or only partially in the annular region 12. For example, the holes 24, 26 can be completely contained within the annular flat cladding layer region 12, or the holes 24, 26 can extend to some extent into the outer cladding layer 36 as shown in the fiber 30 of FIG. . The holes are located adjacent to, aligned with, and in close proximity to the minimum dimension B of the central core 34 (eg, with the hole edges within 3 μm from the central core 34). With respect to alignment, the air holes are substantially aligned with a minimum dimension (B) by a line 28 (FIG. 5) passing through the centers of the holes 24,26. The holes are preferably circular, but can have other shapes as needed, can have equal or unequal dimensions, preferably between about 1-15 μm, more preferably about 5-5. It can have a maximum dimension such as a diameter d (FIG. 5) between 11 μm. Although only one hole is shown on each side, multiple holes along each side can act to produce an elliptical shape and provide a single polarization within the operating wavelength band.

  The outer cladding layer 36 preferably surrounds and is in contact with the annular flat cladding layer region 12. The outer cladding layer 36 preferably has a typical outer diameter of about 125 μm, and preferably has a substantially pure silica composition. If desired, the outer cladding layer 36 can include other suitable dopants such as fluorine, and the outer diameter can be reduced if so specified due to dimensional limitations.

  Schematic views of the relative refractive index profile of the single polarization fiber 30 along the XX and YY axes are shown in FIGS. 3 and 4, respectively. These graphs show the relative refractive index percentage (Δ%) plotted against the fiber radius (in μm) and clearly show the PDL difference in the profile along each such axis. Specifically, the graph shows the maximum relative refractive index Δ1 of the central core 34, the relative refractive index of the hole 26 (truncated due to its depth) and the maximum relative refractive index Δ2 of the annular flat cladding layer region 12. Approximately the relative refractive index of air n air = 1.0, therefore Δ% is largely negative (evaluated to be about −54%). The dashed portion 38 of the profile (shown by dashed line 38—see FIG. 2) represents a fiber 30 in which portion 32 has a circular shape. That is, it should be readily appreciated that the refractive index profiles along each axis are very different. The length of the fiber 30 is designed to be in the range of about 10 cm to 1 m, and a sufficient polarization dependent loss (PDL) difference is greater than 70 dB over a single polarization wavelength range.

  Referring to FIG. 7, another embodiment of a single polarization fiber 30 is shown. The fiber 30 has an elliptical center core 34, circular cross-sectional air holes 24, 26, annular flat clad layer regions 12 and an outer cladding layer region 36 which are arranged on both sides of the short axis of the elliptical core. . In this embodiment, the holes 24 and 16 are partially formed in the annular flat cladding layer region 12 and partially formed in the outer cladding layer 36. The annular flat cladding layer region 12 is sufficiently fluorine doped to provide a Δ% of about −0.4%. The outer cladding layer 36 is preferably made of pure silica. The ranges given above for d (hole diameter), maximum and minimum dimensions A and B, and annular region diameter D are equally suitable for this embodiment.

  Referring to FIG. 8, yet another embodiment of a single polarization fiber 30 is shown. In this embodiment, the fiber 30 has an elliptical central core 34, circular air holes 24, 16 disposed on both sides of the central core so as to lie on the short axis of the elliptical core, and a cladding layer region 22. In this embodiment, the holes 24, 26 are formed in the annular region 12, but the annular region 12 is made of the same material as the cladding layer 22, and this material is preferably pure silica. Dashed line 38 indicates the interface between the core and cladding regions located at a radius greater than the outermost portion of the holes 24,26. In this embodiment of the fiber 30, the cores Δ%, Δ1 are preferably about 1.6% for a single layer clad version of a single polarization fiber laser or amplifier. Although the inner cladding layer 22 is shown as a circle, it can be in the shape of an ellipse or a rectangle with a NA of 0.3 and a cross section of 200 × 400 square μm, as in the other embodiments. Furthermore, if necessary, a dual-clad single polarization fiber laser or amplifier to which an outer cladding layer 36 is added can be implemented.

  Each single polarization fiber 30 according to embodiments of the present invention allows for single polarization (one and only one polarization mode transmission) within the desired SPB 48 (see FIG. 6). It shows the optical characteristics. The SPB 48 of the single polarization fiber 30 according to the present invention is preferably designed to be between about 800-1600 nm. Most preferably, the fiber SPB 48 is designed to match 980, 1060, 1080, 1310 or 1550 nm so that it can be readily used with optical components operating at 980, 1060, 1080, 1310 or 1550 nm. Will. In particular, in accordance with the teachings of the present invention, the SPB center wavelength is selected or tuned by the input mirror or diffraction grating 60 of FIG. It is preferred that they agree (within a range of ± 20 nm). Furthermore, the PZ fiber 30 according to the present invention preferably exhibits an extinction ratio within SPB 48 at 978 nm of preferably 15 dB or more, more preferably 20 dB or more.

Experimental example 1
A first exemplary single polarization fiber 30 according to the present invention having the cross-sectional structure shown in FIG. 7 was made. The fiber 30 has an average diameter d average of about 5.33 μm, a maximum dimension A of about 7.75 μm, a minimum dimension B of about 2.9 μm, so that the first aspect ratio A / B is equal to about 2.7, 1.1% central core Δ%, Δ1 and α have a central core 34 having an α profile of about 2. The holes 24 and 26 are partly included in the annular region 12 and partly included in the cladding layer 22. The average diameter of the holes 24 and 26 is about 8.3 μm. The annular region 12 is doped with fluorine and is thus flat with respect to the pure silica cladding layer 22. The relative refractive index Δ2 of the annular region 12 was −0.4%, and the outer diameter D of the annular region 12 was about 16 μm. In this embodiment, the holes 24, 26 substantially contact the side surface of the central core 34. The tested single polarization fiber 30 exhibited an extinction ratio ER of about 38.6 dB over a length of 1.51 m, for example, at a wavelength of 978 nm. In SPB48, the ER was about 15 dB. The beat length of the fiber length was found to be 4.21 mm. The attenuation measured at 978 nm for a length of 1.45 m was 0.027 dB / m.

Experimental Examples 2 and 3
In Examples 2 and 3, another part along the length of the same fiber (separated from the length of Example 1) was tested and slightly different performance results were obtained. The inventors have determined that this characteristic variation along the length of the fiber is mainly due to process control variations in the prototype fiber that would be well controlled in the production fiber.

Experimental Example 4
Another experimental sample is shown in Table 2 as Experimental Example 4. In this experimental example, the cores Δ% and Δ1 were 2.0%, and Δ2 was −0.4%. In this experimental example, the average core diameter d average ({A + B} / 2) was about 4 μm, and the aspect ratio AR1 was about 3.2. The average hole diameter and other fiber parameters are the same as in Experimental Example 1. As demonstrated in this example, when the relative refractive index of the central core is increased to 2.0%, the single polarization (SP) bandwidth is 42 nm compared to the case where the relative refractive index is 1.1%. Spread to.

The optical properties of the single polarization fiber described above and another experimental fiber are given in Table 2.

  Referring to FIG. 6, the transmission power (dB) versus wavelength (nm) line for the different polarization modes 45, 50 of the fiber 30 shows the single polarization bandwidth for the experimental example 1 fiber of FIG. A graph representing (SPB) is shown. Specifically, the first polarization 45 and the second polarization 50 are measured and plotted as a function of wavelength.

The extinction ratio at 978 nm was determined by passing the optical signal from a 978 nm single wavelength pumping laser with a bandwidth of 0.5 nm through the short fiber length and then measuring the power transmitted at a wavelength of 978 nm. Similarly, ER can be measured in a similar manner within SPB. At the input end, the transmission power associated with the two polarized waves was measured at the output end of the fiber while sequentially aligning the polarizer with one of the birefringence axes. The extinction ratio ER is the formula:
ER = 10logp1 / p2
Was used to determine. here,
p2 is the power in the second polarization,
p1 is the power in the first polarization.

The beat length LB was also measured using the wavelength scanning method by determining the modulation period Δλ and the fiber length L in the spectrum of the light source. Two polarizers were inserted before and after the fiber. Beat length LB is the formula:
LB = {ΔλL} / λ
Is calculated according to Here, λ is the center wavelength (nm) of the light source. In this measurement, a modulation period was obtained by performing a Fourier transform using a broadband ASE source. The wavelength of the ASE source was 970 to 1020 nm, and the center wavelength was 980 nm. The measured beat length was 4.21 mm.

  Similarly, the cutoff wavelength λ1 of the first polarization, the cutoff wavelength λ2 of the second polarization, and the single polarization bandwidth (difference between the cutoff wavelengths of the two polarization modes) were determined. For each measurement, an unpolarized white light source with a flat spectrum at 300-2000 nm was used. Next, a polarizer was inserted into the light incident end, set to two polarization axes determined from the measurement of the extinction ratio, and a cut-off test for each polarization was performed.

The attenuation of a single polarization fiber is determined by measuring the power p1 for the first length of the fiber (approximately 3m), then cutting the fiber to shorten the length (approximately 1m) and measuring the power p2. taking measurement. Then attenuate:
Attenuation = [10logp1-10logp2] / L
Calculate as Here, L is the removed length. Attenuation is measured at 978 nm.

  Referring to FIG. 9, one system 40 using a single polarization fiber 30 in accordance with the single polarization fiber embodiment described herein is shown. System 40 includes an optical device 42, such as a laser, gyroscope, sensor, modulator, beam splitter, polarization multiplexer, etc., having or connected to fiber 30 according to the present invention. The fiber 30 and the optical component 42 can be further contained in a housing 44, and sub-components can be contained in the housing 44.

  Referring to FIG. 10, a system 140 is shown in which a fiber 30 according to an embodiment of the present invention is connected between optical components 42a, 42b, and the fiber and optical components are optionally contained within a housing 44. .

  Referring to FIG. 11, a fiber 30 according to an embodiment of the present invention is connected to an optical component 42, and the fiber 30 is optionally optically coupled to another type of fiber 20 as illustrated in FIG. A system 240 is shown. It will be appreciated that the connection of SP fiber 30 to another type of fiber 20 may be made in various combinations of any order of FIGS. 9-11 as shown in FIG.

  Referring to FIG. 12, to provide a single linearly polarized ytterbium-doped fiber laser, a 10 m fiber section of Yb-doped gain fiber 20 fusion spliced to a passive single-polarization (SP) fiber 30 ′ at 980 nm. Unpolarized pumping light 72 is optically pumped. For the double clad embodiment, the pumping light 72 pumps a laser cavity 46 having a birefringent ytterbium-doped fiber 20 having an elliptical core, spliced to a passive (undoped) single polarization fiber 30 '. To do. A linearly polarized ytterbium-doped fiber laser obtained from optical pumping of a fiber having linear birefringence and linear dichroism formed by a doped fiber section 20 and an undoped SP fiber section 30 'is shown in this exemplary embodiment. The polarization extinction ratio was greater than 30 dB.

  That is, the linearly polarized ytterbium doping is achieved only by optical pumping of a fiber having linear birefringence and linear dichroism formed by the doped fiber section 20 and the undoped SP fiber section 30 'in the forward pumping direction as in FIG. A fiber laser can be obtained. However, additional elements are added to FIG. 12 to show that back reflection is possible. The 500 m polarization-maintaining PM fiber section 20 ′ is a PANDA fiber not doped with ytterbium, commercially available from Corning Incorporated. Single polarization operation is introduced by the incorporation of a polarizing beam splitter 132 and a λ / 2 wave plate 134 in front of the laser cavity 46. A cavity 46 having an elliptical core gain fiber 20 and a single polarization fiber 30 was pumped by unpolarized light from an unpolarized pumping source or pumping laser 72. The measured extinction ratio exceeded 1000: 1.

  A gain fiber 20 of 10 m was doped with 6000 ppm of ytterbium by weight. Birefringence was obtained with an elliptical core with dimensions of 7.9 μm × 3.5 μm. This birefringence corresponded to a group polarization beat length of 7 mm at 1 μm. The pumping laser 72 was a high power (500 mW) single stripe laser diode operating at about 974.5 nm (which was then available from Corning-Lasertron). The pumping light was depolarized by being incident at 45 ° with respect to the polarization axis of the 500 m long polarization maintaining fiber (Corning PM 980) 20 ′ through the beam splitter 132 and the wave plate 134. The degree of polarization of the obtained pumping light was less than 1%.

  The undoped single polarization fiber 30 'had a polarization dependent propagation cutoff wavelength for the fundamental transverse mode. Due to this cut-off difference, there was a wavelength range (similar to the bandwidth in FIG. 6) for only a single polarization to propagate.

  A laser cavity 46 was formed by fusion-bonding a 10 m long section of the Yb-doped elliptical core fiber 20 to the undoped single polarization fiber 30 ′. Because of the polarization-maintaining fiber, the polarization eigen axes of these two fibers were aligned by the splicer 17 (Fujikura 40-PM). Zero transmission was detected by rotating the elliptical core fiber 20 and the single polarization fiber 30 at the junction and rotating the analyzer polarizer 126 at the output. The connection loss between the evaluated elliptical core fiber 20 and the single polarization fiber 30 ′ was less than 1 dB.

  In order to ensure that the operating wavelength of the fiber laser is within the single polarization bandwidth 48 of the single polarization fiber 30 ′, a bulk 1080 nm bandpass filter 60 ′ is placed in the cavity 46. The boundary of the Fabry-Perot fiber cavity 46 was defined at one end by a gold coated high reflector 60 (R> 99.9%) and at the other end by a 3.5% Fresnel reflection at the fiber facet-air interface. The lens 138 is transparent and was used to couple light into and out of the fibers 20 and 30 ′ having linear birefringence and linear dichroism so formed. The reflection is only due to the gold mirror 60 at one end and the fiber-air interface 62 at the pumping light input end.

  The laser output 66 was taken at the pumping end of the fiber laser by placing the microscope cover slide 136 obliquely in the pumping light / signal light path. Care was taken to place it at a very different angle from the Brewster angle to avoid spurious polarization effects.

  The laser produced a power of 50 mW at an incident power of about 150 mW. The cavity is not optimized to obtain the maximum output power. A single polarization fiber was truncated to determine the minimum length required to produce an output containing a high percentage of linearly polarized light.

  Referring to FIG. 13, the polarization extinction ratio as a function of the length of the single polarization fiber 30 is shown. For example, an extinction ratio of 30 dB was obtained using a 5 m long single polarization fiber 30. The output spectrum about each orthogonal polarization axis with respect to the obtained fiber laser is shown in FIG.

  Referring to FIG. 15, the single polarization fiber 30 exhibited a polarization dependent cut-off wavelength. The cutoff wavelength for each polarization eigenmode as a function of fiber length is shown in FIG. The polarization wavelength band of this fiber for a given length is represented by the difference between the two wavelengths (Cutoff 1 and Cutoff 2) in FIG. For example, for a 2 m long fiber, only a single polarization will propagate in this fiber 30 in the wavelength range of 1070 to 1117 nm as a single polarization bandwidth 48 similar to FIG. Therefore, it can be seen from FIG. 14 that the laser gain bandwidth of about 1070 to 1071 nm overlaps the end of the single polarization wavelength range of 1070 to 1117 nm around the wavelength of about 1070 nm selected by the bandpass filter.

  In order to examine the effect of the single polarization fiber 30 ′ as a function of length, the laser was chosen to operate at 1070 nm, which is the short wavelength end of the single polarization passband 48. This 1070 nm wavelength was obtained by rotating the incident angle of a 1080 nm bandpass filter with respect to the propagating beam in the cavity. From FIG. 15, in this case, the length dependency of the polarization dependent loss is larger than the center of the single polarization window 48. As shown in FIG. 13, an extinction ratio greater than 20 dB was obtained with a short single polarization fiber 30 ′ length such as 1 m.

  Thus, linearly polarized fiber lasers can be implemented through the use of undoped single polarization fiber 30 added to doped elliptical core fiber section 20. The measured linear polarization degree exceeded 1000: 1. This fiber laser can be further optimized through the use of a fiber grating to provide more subtle wavelength selective feedback and to tune the gain bandwidth to overlap the single polarization wavelength range at various locations. it can.

  It will be apparent to those skilled in the art that variations and modifications can be made to the present invention without departing from the scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of the invention provided they come within the scope of the appended claims and their permeates.

  Some portions of this invention were made with US government support under Contract No. MDA-972-02-3-004 granted to DARPA. The United States government may have certain rights in some of the claims of the present invention.

In accordance with the present invention, a single polarization that is optically coupled to an optical component, such as an active pumping light source 42 for use as a fiber laser, or used as an active pumping light source 42 itself for use as an amplifier. 1 is a schematic diagram of a system comprising a wave active optical fiber; 2 is a cross-sectional view of a first embodiment of the single-polarization active optical fiber 30 of FIG. 1 in accordance with the present invention. FIG. FIG. 3 is a conceptual diagram of a refractive index profile of the first embodiment taken along axis XX of FIG. 2 in accordance with the present invention. FIG. 3 is a conceptual diagram of a refractive index profile of the first embodiment taken along the axis YY of FIG. 2 in accordance with the present invention. 1 is an enlarged partial cross-sectional view of a first embodiment of a single polarization optical fiber according to the present invention. FIG. Exemplary diffraction of an input mirror 60 centered at the cutoff wavelength of the second polarization 50 within the gain bandwidth 650 of the single polarization optical fiber 30 operating as the laser of FIG. 1 in accordance with the present invention. FIG. 4 is a graph illustrating a representative wavelength band of a single polarization of an embodiment of a single polarization optical fiber 30 aligned with a graph illustrating the grating filter bandwidth. FIG. 3 is a cross-sectional view of a second embodiment of single polarization optical fiber 30 of FIG. 1 in accordance with an embodiment of the present invention. FIG. 4 is a cross-sectional view of a third embodiment of single polarization optical fiber 30 of FIG. 1 in accordance with an embodiment of the present invention. 1 is a schematic diagram of a system comprising a single-polarization active optical fiber 30 according to an embodiment of the invention optically coupled to an optical component. 1 is a schematic diagram of a system comprising a single-polarization active optical fiber 30 according to an embodiment of the invention optically coupled to an optical component. 1 is a schematic diagram of a system comprising a single-polarization active optical fiber 30 according to an embodiment of the invention optically coupled to an optical component. A single polarization having a doped elliptical core in the fiber section to form a fiber having optically active birefringence and linear dichroism according to another embodiment of the invention optically coupled to an optical component. 1 is a schematic illustration of a laser cavity including an undoped plate 30 ′ of a wave optical fiber. Fig. 13 is a graph of extinction ratio as a function of length for the single polarization fiber 30 'of Fig. 12 in accordance with the present invention. FIG. 13 is a graph of output spectrum versus orthogonal polarization axis for the laser of FIG. 12 in accordance with the present invention. FIG. 13 is a graph of the fundamental mode cutoff spectrum for each polarization of the single polarization fiber 30 ′ of FIG. 12 in accordance with the present invention. FIG. 16 is a graph of the fundamental mode cutoff spectrum for each polarization of the single polarization fiber 30 ′ of FIG. 15 as a function of length in accordance with the present invention.

Explanation of symbols

12 circular flat clad layer region 20 single mode fiber 22 core / cladding interface 24,26 hole 30 single polarization fiber 34 core 48 single polarization wavelength range 90 active dopant 650 operating wavelength range

Claims (7)

An optical active single linear polarization device, wherein the optical active single linear polarization device is
An optical waveguide having a single polarization wavelength range for propagating light, having linear birefringence and linear dichroism, and an operation of the waveguide in an operating wavelength range overlapping the single polarization wavelength range; A plurality of active dopants disposed on a portion of the optical waveguide having linear birefringence and linear dichroism to provide,
Which has
The waveguide includes a polarization maintaining (PM) fiber having an optical fiber polarization component associated with the first linear polarization eigenmode and an optical fiber polarization component associated with the second linear polarization eigenmode. A wave dependent loss (PDL) difference is accumulated between the first polarization mode and the second polarization mode over a waveguide length, and the first polarization mode is first at a first cutoff wavelength. And the second polarization mode has a second 3 dB attenuation at a second cutoff wavelength, and therefore between the first cutoff wavelength and the second cutoff wavelength. Providing a single polarization wavelength range having a single polarization center wavelength, wherein the first cutoff wavelength is less than the second cutoff wavelength, and wherein the single polarization center wavelength is the center of the operating wavelength range; Close to the wavelength,
The optical fiber has an optically active central core having a substantially elliptical shape, and the optical fiber has at least one air hole disposed on each side of the central core ; Each edge is located within 3 μm from the central core, and a line passing through the center of the air hole is substantially aligned with an axis having the minimum dimension of the central core , and the optical fiber is in the operating wavelength range. Supporting a single polarization mode within, and
A single polarization device characterized in that the waveguide length is in the range of 5 cm to 1 m and the polarization dependent loss (PDL) difference is greater than 3 dB over the single polarization wavelength range.   A pump signal coupled to the waveguide for exciting the plurality of active dopants, the plurality of active dopants for emitting output light in the operating wavelength range; The single polarization device according to claim 1, wherein a gain medium is provided.   The single polarization device further includes a wavelength selection filter, and the output light emitted from the gain medium is fed back over the predetermined narrowband wavelength range by the predetermined narrowband wavelength range of the wavelength selective filter. The single polarization of claim 2, wherein the predetermined narrowband wavelength range is encompassed within the single polarization wavelength range. Wave device. The single polarization device according to claim 1, wherein the optical waveguide having linear birefringence and linear dichroism includes a high birefringence fiber having birefringence greater than 10 ^-6 .   2. The single polarization device according to claim 1, wherein the optical waveguide having linear birefringence and linear dichroism includes a gain-doped elliptical core fiber connected to an undoped single polarization fiber.   4. A single polarization device according to claim 3, wherein the wavelength selective filter comprises a fiber Bragg grating.   2. A system comprising a single polarization device according to claim 1, wherein providing the operation of the waveguide comprises providing a gain.

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